fix: extract handleNodeDoubleClick into useCallback and add tests

Replace inline arrow with named useCallback handler to avoid creating
a new fn ref on every render. Add two CanvasContainer tests covering
the double-click callback and the no-op path when prop is omitted.

frontend/src/App.tsx

@@ -343,6 +343,10 @@ export default function App() {
     setEditEdgeId(edge.id)
   }, [])
 
+  const handleNodeDoubleClick = useCallback((node: Node<NodeData>) => {
+    handleEditNode(node.id)
+  }, [handleEditNode])
+
   const handleEdgeUpdate = useCallback((data: EdgeData) => {
     if (!editEdgeId) return
     snapshotHistory()
@@ -400,7 +404,7 @@ export default function App() {
                 <CanvasContainer
                   onConnect={handleEdgeConnect}
                   onEdgeDoubleClick={handleEdgeDoubleClick}
-                  onNodeDoubleClick={(node) => handleEditNode(node.id)}
+                  onNodeDoubleClick={handleNodeDoubleClick}
                   onNodeDragStart={snapshotHistory}
                   onOpenPending={(deviceId) => {
                     setHighlightPendingId(undefined)

frontend/src/components/canvas/__tests__/CanvasContainer.test.tsx

@@ -104,6 +104,24 @@ describe('CanvasContainer', () => {
     }).not.toThrow()
   })
 
+  // ── Node double-click ─────────────────────────────────────────────────────
+
+  it('calls onNodeDoubleClick prop when a node is double-clicked', () => {
+    const onNodeDoubleClick = vi.fn()
+    const node = makeNode('n1')
+    render(<CanvasContainer onNodeDoubleClick={onNodeDoubleClick} />)
+    ;(rfProps.onNodeDoubleClick as (...args: unknown[]) => unknown)({} as MouseEvent, node)
+    expect(onNodeDoubleClick).toHaveBeenCalledWith(node)
+  })
+
+  it('does not throw when onNodeDoubleClick is not provided', () => {
+    const node = makeNode('n1')
+    render(<CanvasContainer />)
+    expect(() => {
+      ;(rfProps.onNodeDoubleClick as (...args: unknown[]) => unknown)({} as MouseEvent, node)
+    }).not.toThrow()
+  })
+
   // ── Connection validation ─────────────────────────────────────────────────
 
   it('isValidConnection returns false for self-connections', () => {